Wireless mobile access to the Internet is expected to represent an increasingly important segment of the communications industry. Access to the Internet will be offered, for example for wireless connection enabled laptop and palmtop computers, by a variety of wireless access network operators. Wireless access networks typically have a number of wireless base stations and mobile hosts, e.g. wireless enabled mobile computers acting as mobile Internet hosts, can connect to one or more of these base stations at any time and use it or them to relay data to/from a correspondent host or hosts, for example in the wired network. Due to mobility, a mobile host may sometimes need to release its connection to a base station and establish a connection to another base station. If this action happens during an active data session, the network must ensure that there is little disturbance to the ongoing communication between the mobile host and the correspondent host or hosts. The mechanisms that support such a mobility of a mobile host are referred to as handover.
The geographical areas covered by different access networks often overlap, thus allowing for quick migration from one network to another one. A mobile host may sometimes need to perform a handover between base stations that belong to two different networks. The term “inter-network handover” is used herein to refer to a mobile host's moving from one access network to another access network during an active data session. Due to inherent characteristics of the Internet Protocol (IP) a movement of a mobile host between different access networks necessarily involves a change of the mobile host's IP address. However, a change of an Internet host's IP address during an active data session breaks the ongoing session. Special mechanisms are therefore required that allow mobile hosts to move between access networks during active data sessions.
The so-called Mobile IP protocol represents a mechanism that allows a mobile host to maintain an active data session during migration from one access network to another, even though the mobile host needs to change its IP address. In this solution a static anchor point (referred to as Home Agent) is assigned to mobile hosts. Correspondent hosts willing to send packets to a mobile host need to transmit packets to its Home Agent which, in turn, forwards packets to the mobile host's actual location. When the mobile host moves to a new network and is assigned a new IP address, it must inform its Home Agent. In a variant of this protocol, referred to as Mobile IP with Route Optimization, the correspondent hosts or nodes are allowed to send packets directly to the mobile host's actual location thus eliminating need to use Home Agents for the forwarding of each packet. The first (or first few) packets of a data session, however, are still forwarded to the mobile host through its Home Agent. In this variant of the Mobile IP protocol, mobile hosts must also notify correspondent hosts after migrations from one access network to another.
The drawback of the Mobile IP protocol, with or without Route Optimization, is the need for a statically allocated Home Agent. The use of Home Agent ties the operation of a mobile host to its home network even when its actual location is a long distance from its home. Messages exchanged between the mobile host and its Home Agent may travel a long way and consume a significant amount of network resources in addition to suffering delay. The Route Optimization variant of Mobile IP removes the need to forward all packets through the Home Agent but it requires, in exchange, a security association between the mobile host and a correspondent host. In addition, it does not eliminate the need to inform the Home Agent when migration between access networks takes place, since communication sessions still start by forwarding packets through the Home Agent.
Home Agents can be avoided if correspondent hosts send their packets directly to a mobile host's actual IP address at all times. A mechanism that supports this has been proposed which uses a (possibly distributed) database that maps Network Access Identifiers (NAI) of mobile users to IP addresses. Herein, this database will be referred to as User Name Service (UNS). Correspondent hosts willing to communicate with a mobile host can contact this database and obtain the actual IP address of their called party, i.e. the mobile host. Using this mechanism, however, a mobile host cannot maintain its active data session when it moves to a new network and changes its IP address. The mechanism only supports migrations between data sessions.